Polycarbonates are useful in the manufacture of articles and components for a wide range of applications, from automotive parts to electronic appliances. Because of their broad use, particularly in electronic applications, it is desirable to provide polycarbonates with flame retardancy. Many known flame retardant agents used with polycarbonates contain bromine and/or chlorine. Brominated and/or chlorinated flame retardant agents are less desirable because impurities and/or by-products arising from these agents can corrode the equipment associated with manufacture and use of the polycarbonates. Brominated and/or chlorinated flame retardant agents are also increasingly subject to regulatory restriction.
Nonhalogenated flame retardants have been proposed for polycarbonates, including various fillers, phosphorus-containing compounds, and certain salts. It has been difficult to meet the strictest standards of flame retardancy using the foregoing flame retardants, however, without also using brominated and/or chlorinated flame retardants, particularly in thin samples.
Polysiloxane-polycarbonate copolymers have also been proposed for use as non-brominated and non-chlorinated flame-retardants. For example, U.S. Application Publication No. 2003/0105226 to Cella discloses a polysiloxane-modified polycarbonate including polysiloxane units and polycarbonate units, wherein the polysiloxane segments include 1 to 20 polysiloxane units. Use of other polysiloxane-modified polycarbonates are described in U.S. Pat. No. 5,380,795 to Gosen, U.S. Pat. No. 4,756,701 to Kress et al., U.S. Pat. No. 5,488,086 to Umeda et al., and EP 0 692 522B1 to Nodera, et al., for example.
While the foregoing flame-retardants are suitable for their intended purposes, there nonetheless remains a continuing desire in the industry for continued improvement in flame retardance. One need is for articles that are not as prone to burn-through, that is, the formation of holes upon the application of a flame. Thin articles in particular present a challenge, since bum-through holes tend to form more quickly. Non-brominated and/or non-chlorinated flame-retardants can also adversely affect desirable physical properties of the polycarbonate compositions, particularly impact strength.
Also, when polycarbonates are used in electronic applications, many types of electrical equipment produce stray electromagnetic radiation, referred to as electromagnetic interference (EMI). EMI may occur, for example, from analog circuit components or from digital components. EMI emissions are undesirable since they can potentially interfere with the operation of nearby electrical equipment. In addition, regulations (such as the EMC (Electromagnetic Compatibility) regulations) have been established for the maximum permissible EMI emissions from various types of electrical equipment, and these regulations must be taken into account when designing new equipment in which EMI might be a problem.
There are many prior art methods that have been used for EMI shielding purposes, such as electroless plating, conductive painting, vacuum metallising etc. However, all of these methods and/or processes are less desirable due to the disadvantages associated therewith such as environmentally unfriendly processes, poor adhesion between plastics and conductive layer, difficult to handle complex shapes, and the like.
Accordingly, it would be beneficial to provide a thermoplastic material that offers improved flame retardance without use of brominated and/or chlorinated flame-retardants. It would also be beneficial to provide a thermoplastic material that offers improved EMI shielding such that these thermoplastic material may have greater utility in electronic applications. It would also be beneficial if improved flame retardance and EMI shielding could be achieved without substantial degradation of properties such as impact strength.